The present invention relates generally to a medical diagnostic measurement instrument, and, more specifically, to a device and method for obtaining non-invasive quantitative measurements of blood glucose in patients.
The frequent monitoring of blood glucose levels in individuals with diabetes mellitus has become a major factor in the care of such patients over the past decade. Currently, it is possible for the diabetes patient and health care professionals to measure and record blood glucose levels using a variety of portable devices. Due to the need for multiple daily measurements, invasive blood for samples are a burden on the patient and often expensive. As a result, non-invasive devices using spectroscopic techniques, and which are battery powered and use solid-state electronics, have begun to be commercialized. Used at home, these devices allow diabetes patients to monitor and respond to fluctuations in blood glucose on a daily basis.
One example of such a device is disclosed in U.S. Pat. No. 5,070,874 to Barnes, et al. (xe2x80x9cthe ""874 patentxe2x80x9d). As set forth in the ""874 patent, human blood glucose concentration levels vary greatly, and are found within the range of 0-600 milligrams per deciliter (mg/dl). Normal human blood glucose levels are in the approximate range of 80-110 mg/dl. Devices of the type disclosed in the ""874 patent involve measurement of blood components using near infrared radiation and spectroscopic absorption techniques. Additional devices of this type are disclosed in U.S. Pat. Nos. 5,379,764 and 4,882,492, as well as numerous others, which make use of both reflectance and transmission spectroscopic analysis techniques.
Problems with these prior art devices have resulted due to several issues. One problem is the overlap of the spectrum of glucose with other blood sugars and chemicals. Another relates to hemoglobin-glucose binding, which renders discrete spectral measurements difficult. Also, spectroscopic techniques are typically unable to discriminate between sugars that are metabolized and those that are excreted, resulting in erroneous readings. Still further, prior art devices have failed to address issues which directly impact the accuracy of the measurements taken, such as the spectral effect produced by the skin and tissue, as well as variable blood vessel and skin thickness and composition.
As a result of these and other problems, the repeatability and resulting accuracy of such devices has not been in the range it is desired. The U.S. Food and Drug Administration is currently advising that non-invasive glucose measuring devices should have an accuracy in the range of 15% error or less.
According to the present invention, a transmission glucose measuring device is provided which uses a signal sensor assembly to illuminate intravascular blood or fluid components in the body. The assembly includes near infrared light sources on an external surface of a translucent body to illuminate blood or fluid, and light receptors positioned on an opposite external surface of the same body, to receive respective signals representing the radiation transmittance through the tissue and blood or fluid components illuminated. In the preferred embodiment one (1) light source emitting near infrared light of approximately 940 nm wavelengths is used. Alternatively, additional light sources, from two (2) to four (4) light sources, may be used emitting near infrared and infrared light between preferably 640 and 1330 nm, more preferably 650 and 1300 nm wavelengths. A fifth (5) possible light source may also be used, which would repeat one of the previous four (4) wavelengths. The light sources in the preferred embodiment are light emitting diodes (LEDs) which are pulsed at 1 kiloHertz (kHz) for a 1 millisecond (ms) pulse width. Where more than one receptor, or sensor, is used, each operates at a time when the other receptors are off to avoid further noise and signal contamination. The LEDs and opposite receptors are mounted on a biased or spring biased support for convenient attachment of the LEDs and receptors to the body part. In the preferred embodiment, a spring biased support is used for mounting on external surfaces of the human ear. The range of pressure applied to the ear by such supports and the associated LEDs and receptors should be no more than 15-30 mm of mercury (Hg), and is preferably much less, for example, 0.4 oz/square inch. It is understood that numerous shapes and configurations for the support could be used, depending on the shape of the body or body part to be measured.
Prior to use of the device, upon turning the device on a self-check is performed to ensure that all LEDs and respective receptors are operating to specification. Prior to use of the device, a calibration process is conducted to establish settings within the device which consider the skin or tissue and blood flow characteristics of the subject. Such calibration is believed to enable improved accuracy and predictability in glucose measurement in the present device, since factors such as tissue thickness and composition, as well as blood flow, are taken into consideration. Intensity calibration involves setting the intensity of the LEDs based on an LED intensity factor which is derived from the high and low data values measured from a pulse waveform signal.
The pulse of the subject is measured using an LED and its associated receptor to obtain the pulse waveform signal. The high and low blood flow data values collected to obtain the pulse waveform signal of the subject are converted and stored in a digital processor, such as an LED signal processor. Once the high and low pulse waveform signal values are known, the blood flow characteristics of the subject are used for the intensity calibration.
The intensity factor may be established based upon initial readings of the pulse waveform signal. Current is increasingly supplied to the LED to increase the intensity of the light source in a stepped fashion at one of multiple increments, until a minimally distorted desirable signal is received by the receptor. Once an acceptable signal is received, this selected level of LED intensity is stored by the processor, and becomes the level of current applied to each LED during operation of the device. Additionally, each LED is operated to determine that it is properly operational and that its respective receptor is receiving the LED""s signal at the desired LED intensity. Alternately, the LED intensity factor may be established using a baseline voltage of approximately 1.2 volts. The LEDs are continuously checked by the device to ensure proper operation. In the event no signal is received, the device prevents a measurement from being taken and issues a warning notice to the operator.
Still another step in device calibration involves determining from the pulse waveform signal when measurements or readings should be taken by the device. Measurements of the LED signal are preferably only taken at a midpoint in the blood flow cycle, or at thexe2x80x9cbaselinexe2x80x9d of the pulse waveform signal. For example, the difference between the high and low data values from the pulse waveform signal result in a value which is provided to the signal processor for establishing the timing of measurements, or signal generation, taken by the device with respect to the blood flow of the subject. Once the LED intensity factor and the baseline of the pulse waveform signal are determined, the device then initiates the operation and measurement of each of the LED signals, preferably through an ear lobe of the subject. Measurements from each LED are preferably taken several predetermined times, for example 30 seconds, at each of the high and low pulsatile values measured over 5 milliseconds, with the resulting sensor signal values stored and amplified in a sample and hold amplifier in the LED signal processor, converted in an analog-to-digital (A/D) converter, and normalized and averaged to obtain a single digital data value for each LED signal.
This pre-processed digital signal value from the LED signal processor is then provided to a further digital processor, preferably via a personal computer interface of the type well known to those of skill in the art. The digital processor is preferably a personal computer supporting conventional software and a database containing predetermined or target spectral glucose transmittance and absorbance data over a range of 0 to 600 mg/dl, for determining the glucose level of the subject from the digital signal value provided. Alternatively, a trained neural network containing the predetermined or target spectral glucose data may be used. The pre-processed digital signal value is incrementally compared to target data values in the database orxe2x80x9clook-up tablexe2x80x9d to obtain a value which is slightly higher than the pre-processed signal value. Alternatively, the database could be used, and the comparison made in a trained neural network which is also well known to one of skill in the art. This closest incremental value, which is calculated by a linear interpolation between database values if no specific value is located within the database, is then provided to a digital display as the glucose level for review by the subject.
Other features and advantages the present device will become apparent from the following detailed description of the preferred embodiment made with reference to the accompanying drawings, which form a part of the specification.